Method for seismic monitoring of an underground zone by simultaneous use of sererval vibroseismic sources

ABSTRACT

Method and system intended for seismic monitoring of an underground zone ( 1 ), comprising simultaneously using several seismic vibrators.  
     The system comprises for example several local units (LU) comprising each a vibrator ( 5 ), a seismic pickup antenna ( 2 ), a local acquisition and processing unit ( 6 ), and a central control and synchronization unit ( 8 ) for simultaneously controlling the various vibrators by means of orthogonal signals, local units ( 6 ) being suited, by means of particular processing, to isolate and to reconstitute the seismograms corresponding to the contributions of the various vibrators.  
     Applications: monitoring of a hydrocarbon reservoir during production or of a reservoir used for gas storage for example.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and to a deviceintended for seismic monitoring of an underground zone such as areservoir, comprising simultaneously using several seismic vibrators.

BACKGROUND OF THE INVENTION

[0002] It is well-known to monitor the long-term state variations of areservoir during production, either a hydrocarbon reservoir or areservoir intended for gas storage, by means of a seismic systemcomprising an impulsive seismic source or a seismic vibrator emittingseismic waves in the ground and a reception device comprising seismicpickups arranged at the surface or in wells and coupled with theformations to be monitored. At predetermined time intervals, seismicinvestigations are carried out with wave emission, reception of thewaves reflected by the subsoil discontinuities and recording ofseismograms, so as to determine by comparison the changes that takeplace in the reservoir as a result of the development thereof.

[0003] Various long-term seismic monitoring systems are described forexample in patents EP-591,037 (U.S. Pat. No. 5,461,594), FR-2,593,292(U.S. Pat. No. 4,775,009), FR-2,728,973 (U.S. Pat. No. 5,724,311) orFR-2,775,349.

[0004] Patents FR-2,728,973 and FR-2,775,349 notably describe systemsintended for seismic monitoring of an underground zone duringdevelopment, either a hydrocarbon reservoir or a gas storage reservoirfor example. As diagrammatically shown in FIGS. 1 to 3, they comprisefor example a network of seismic antennas 2 consisting each of a seriesof seismic pickups 4 arranged at regular intervals along a well 3drilled in the ground. This network can be regular as shown in FIG. 2,or irregular. The pickups can be one-directional geophones orientedvertically or multi-axis geophones (triphones) and/or hydrophones. Aseismic source 5 is arranged in the vicinity of each antenna 2.Piezoelectric type vibrators such as those described in patentapplication FR-99/04,001 filed in the joint names of the applicants areadvantageously used as sources and permanently installed in theimmediate neighbourhood of each antenna 2.

[0005] The seismic waves generated by the or by each seismic source 5are propagated downwards (downgoing waves 9). These incident waves arefirst recorded by receivers 4 in each well 3. The waves reflected by thediscontinuities of the zone (seismic interfaces) are propagated upwards.These upgoing waves 10 are also recorded by the various receivers 4. Theupgoing waves and the downgoing waves are thus superimposed on theseismograms. They are usually processed by means of a method similar tothe VSP (Vertical Seismic Profiles) processing method well-known to themanskilled in the art.

[0006] The various sources of the seismic system can be actuatedsuccessively by providing, between each triggering, a sufficient timeinterval for reception of the waves reflected by the investigated zone.Several seismic sources emitting the same signals can also be used andtriggered simultaneously in order to increase the power emitted.

[0007] Patent FR-2,589,587 (U.S. Pat. No. 4,780,856) also describes amarine seismic prospecting method comprising emission of seismic wavesby a vibrator or simultaneously by several vibrators controlled by codedvibrational signals according to a pseudo-random code.

SUMMARY OF THE INVENTION

[0008] The method according to the invention allows to carry outoperations intended for seismic monitoring of an underground formation.It comprises:

[0009] emission of seismic waves in the formation by coupling with theformation at least two vibrators emitting simultaneously and controlledby orthogonal signals so as to form a composite vibrational signal 1,

[0010] reception of the signals reflected by the formation in responseto the emission of seismic waves,

[0011] recording the signals received by at least one seismic pickup,and

[0012] formation of seismograms by processing the signals recorded,comprising discrimination of the respective contributions of thevibrators to the composite vibrational signal and reconstruction ofseismograms equivalent to those that would be obtained by actuating thevibrators separately.

[0013] Sinusoidal signals of different frequencies, in their fundamentalcomponents as well as in their respective harmonics, or signals based onwavelets, on Legendre polynomials or on random series, etc, are forexample used as orthogonal signals.

[0014] In the case notably where the orthogonal signals emitted aresinusoids, discrimination of the respective contributions of thevibrators is for example carried out by determining the amplitude andthe phase of the composite vibrational signal at the fundamentalfrequencies of the pilot signals applied to the vibrators.

[0015] Discrimination of the respective contributions of the vibratorscomprises for example weighting the recorded signal by a bell weighting(or tapering) factor and determining the amplitude and the phase of thecomposite signal.

[0016] In order to carry out discrimination of the respectivecontributions of the vibrators, a selection by Fourier transform of thelines of the complex spectrum respectively associated with the variousweighted signals is for example performed.

[0017] Reconstruction of the seismograms specifically corresponding tothe various vibrators is performed for example by applying, afterseparation thereof, an inverse Fourier transform to the linesrespectively associated with the various weighted signals.

[0018] According to an implementation mode, the frequencies of theorthogonal pilot signals respectively applied to the various vibratorsare shifted by frequency intervals, at predetermined time intervals, soas to sweep a certain emission frequency band.

[0019] The system intended for seismic monitoring of an undergroundformation according to the invention comprises means allowing emissionof seismic vibrations in the formation comprising at least two vibratorsand means for generating orthogonal signals and for applying themrespectively to the vibrators so as to generate in the formation acomposite vibrational signal, means allowing reception of the signalsreflected by the formation in response to the emission of seismic waves,means for recording the signals received by the reception means andmeans for processing the signals recorded in order to form seismograms,comprising at least one computer suited to carry out discrimination ofthe respective contributions of the vibrators to the compositevibrational signal and reconstruction of seismograms equivalent to thosethat would be obtained by separately actuating the vibrators.

[0020] According to a first implementation mode, the system comprises atleast two local units arranged at a distance from one another andcoupled with the formation, each unit comprising at least one seismicpickup, a seismic vibrator, a local device intended for acquisition andprocessing of the signals received, and a central control andsynchronization unit connected to the various units, comprising agenerator suited to apply to the vibrators the orthogonal vibrationalpilot signals.

[0021] According to another implementation mode, the system comprises atleast two local units arranged at a distance from one another andcoupled with the formation, each unit comprising at least one seismicpickup, a seismic vibrator, and a central control and synchronizationunit connected to the various local units by a material (cables forexample) or immaterial link (radio) and comprising a signal generatorsuited to form the various orthogonal vibrational pilot signals, andmeans intended for acquisition of the signals received by the variousantennas and for reconstruction of seismograms corresponding to thecontributions of the various vibrators.

[0022] The reception means comprise for example at least one antennaconsisting of several seismic pickups arranged along a well drilled inthe formation, this antenna being connected to the recording means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Other features and advantages of the method and of the systemaccording to the invention will be clear from reading the descriptionhereafter of non limitative examples, with reference to the accompanyingdrawings wherein:

[0024]FIG. 1 diagrammatically shows a system intended for monitoring ofan underground formation, comprising several signal emission andacquisition units,

[0025]FIG. 2 shows an example of distribution, at the surface, ofmonitoring devices,

[0026]FIG. 3 diagrammatically shows a signal emission and acquisitionunit comprising seismic pickups arranged so as to form antennas,

[0027]FIG. 4 shows a variant of the monitoring system of FIG. 1 wherethe seismic signal acquisition means are centralized in a centralstation,

[0028]FIG. 5 illustrates the various stages of the algorithm forimplementing the method, and

[0029]FIG. 6 diagrammatically shows the path of the waves between twoemission points X1, X2 and a common reception point.

DETAILED DESCRIPTION

[0030] The method thus allows to carry out seismic monitoring operationsin an underground zone by using a series of seismic pickups and aplurality of vibrators simultaneously actuated by signals at differentfrequencies selected so as to allow discrimination of the contributionsof each source on the seismograms formed from the signals received andrecorded. This is generally done through control of the various sourcesby <<orthogonal>>signals representing functions referred to asorthogonal functions, well-known to the man skilled in the art, and byusing well-known numerical calculation techniques such as the inverseFourier transform for separating the contributions to the seismogramsobtained of the various vibrators, as explained hereafter by means ofthe following notations: Convolution * Correlation ★ Emission lengtht_(s) (seconds) Listening period t_(e) (seconds) Sampling interval t_(i)(seconds) Initial frequency f_(b) (Hertz) Final frequency f_(f) (Hertz)Elementary frequency f_(l) = 1/t_(e) (Hertz) Line width f_(d) (Hertz)

[0031] A—Orthogonal functions

[0032] We consider two unit sinusoidal signals P₁ and P₂ of respectivefrequencies f₁ and f₂ emitted by two sources S₁ and S₂ located at pointsX₁ and X₂ (FIG. 6) for a duration t_(s) whose value is great compared to1/f₁ and 1/f₂. P₁ = sin2πf₁t P₂ = sin2πf₂t.

[0033] The recorded seismogram of the signals received at a receptionpoint R from source S₁ emitting alone is: T₁=A₁ sin (2πf₁t−Φ₁), where Φ₁is a phase lag.

[0034] Similarly, the seismogram observed at the same point R from S₂emitting alone is: T₂=A₂. sin (2πf₂t−Φ₂), where Φ₂ is also a phase lag.

[0035] If S₁ and S₂ emit simultaneously, the linearity of thetransmission of the seismic waves implies that the seismogram of thewaves received at R is the sum of T₁ and T₂.

[0036] Furthermore, if f₁≠f₂, P₂ ★ P₁ = 0 (A), T ★ P₁ = T₁ ★ P₁ (B), andT ★ P₂ = T₂ ★ P₂ (C).

[0037] Equation (A) expresses the orthogonality of signals P₁ and P₂;equations (B) and (C) express the possibility of separating compositesignal T into its two components. This property theoretically applies toany number of sources emitting sinusoids of different frequencies ormore precisely orthogonal signals but, in practice, the number ofsources has to be limited because of the following phenomena:

[0038] a) the distortion which cannot be disregarded with mechanicalsources. As it emits frequency f₁, source S₁ also emits frequencies 2f₁,3f₁ . . . nf₁. Consequently, if f_(i) and f_(j) are the respectivefrequencies of the two sources S_(i) and S_(j) of the array of sources,we must have f_(i)≠f_(j), as well as f_(i)≠2f_(j), f_(i)≠3f_(j), . . .f_(i)≠nf_(j);

[0039] b) the necessarily truncated nature of emission length (t_(s)),which is expressed in the frequency domain by means of a convolution ofthe line (impulse) by the Fourier transform of the truncation. If thelatter is sudden (multiplication by a boxcar of length t_(s)), it is adiffraction function of great width If it is progressive (multiplicationby a bell-shaped curve, a Gaussian curve or a Hanning function forexample), it is another bell function whose width is inverselyproportional to the length of the truncation, and

[0040] c) the imperfection of the sources, which affects their stabilityand the precision of the frequencies emitted. In practice, it can beconsidered that this imperfection simply contributes to the increase inthe line width.

[0041] The simplest orthogonal functions are sinusoids of differentfrequencies. Other orthogonal functions can also be used: functionsbased on Legendre polynomials, wavelets, random series, etc.

[0042] B—Reversibility of the Fourier Transform

[0043] Instead of emitting a sinusoid T_(i) of frequency f_(i), ofamplitude A_(i) and of phase Φ_(i), if one emits composite signal P_(t)consisting of the sum of N sinusoids {f_(i), A_(i), Φ_(i)} with 1≦i≦N,all the frequencies being contained in a spectral band contained betweentwo limit frequencies f_(b) and f_(f), the seismogram T_(t) observed atpoint R will have as the Fourier transform at frequency f_(i) the numberof amplitude A_(i) and of phase Φ_(i) equal to the amplitude and to thephase of sinusoid T_(i). It is thus possible, by successively emittingall the sinusoids of frequencies f_(b) to f_(f), to reconstructseismogram T_(t) by inverse Fourier transform.

[0044] In cases where, for example, all the amplitudes A_(i) are equalto 1 and all the phases Φ_(i)=0, the signal P_(t) obtained is very closeto the signal resulting from cross-correlation of a sliding-frequencysignal contained in the [f_(b)−f_(f)] range (sweep), commonly used invibroseismic methods. According to the discrete Fourier transformtheory, well-known to the man skilled in the art, if it is desired tolisten, to source S_(t) during the time t_(e), the frequency incrementbetween the sinusoids is Δƒ=1/t_(e) and the number of sinusoids requiredis N_(f)=(f_(f)−f_(b))t_(e).

[0045] N vibrators installed in the field can thus be excitedsimultaneously by means of vibrational signals whose frequencies aresuch that each source is successively excited by each one of the N_(f)sinusoids above at any time, on condition that the respectivefrequencies of the sinusoids emitted at the same time by the variousvibrators are all different. Separation of the signals received by thepickups in the field, in response to the simultaneous emission of thevarious signals, is thus obtained by selection of the line at thesuitable frequency.

[0046]FIG. 5 diagrammatically illustrates the various stages of themethod. Sinusoidal pilot signals 11 of respective frequencies af₀, bf₀,cf₀, df₀, etc., are simultaneously applied to the various seismicsources 5 installed in the field, coefficients a, b, c, d, etc., beingso selected that these frequencies are different from one another anddifferent from their respective harmonics. These frequencies are wholemultiples of a fundamental frequency f₀.

[0047] The seismogram 12 that is obtained by recording the wavesreceived by the pickups of the various antennas 4 is a linearcombination of the seismograms that would have been obtained by excitingsources 5 sequentially.

[0048] The recorded signals are then weighted by multiplying them by abell weighting factor referred to as tapering factor 13 in order to formtapered or weighted signals 14. The real part 15 and the imaginary part16 of the Fourier transform of the tapered signals are then calculated.Each part consists of impulses separate from one another. For eachsource 5, only the real number 17 and the imaginary number 18 formingthe complex value of the Fourier transform at the frequency emitted bythe source are then kept.

[0049] The sets of various numbers 17 and 18 when the source emits allthe programmed frequencies form the real part 19 and the imaginary part20 of the seismogram 21 associated with the source. This seismogram isobtained by inverse Fourier transform.

[0050] According to a first example of implementation of the method, thesystem comprises a plurality of local units LU comprising each anantenna 2 connected by cables (not shown) and a local acquisition andprocessing device 6 (FIGS. 1, 2), and the various vibrators areconnected by cables C for example to a central control andsynchronization unit 8 comprising a signal generator (not shown) suitedto generate, for the various vibrators 5, the orthogonal pilot signalsdefined above.

[0051] According to another implementation mode (FIG. 4), the variousreception antennas 2 are connected by cables C for example to centralcontrol and synchronization unit 8 which fulfils the tasks of generationof the composite signals for the various sources 5 and acquisition andrecording of the signals received by pickups 4, as well as processing ofthe acquired signals.

[0052] Of course, cables C can in general be replaced by any material orimmaterial link (radio link, optical fiber, etc.).

[0053] Local acquisiton and processing devices 6 and/or central controland synchronization unit 8 comprise computers such as PCs programmed tocarry out processings intended to isolate and to reconstitute theseismograms corresponding to the specific contributions of the variousvibrators 5 as defined in the description.

1) A method intended for seismic monitoring of an underground formation(1) comprising emission of seismic waves in the formation, reception ofthe signals reflected by the formation in response to the emission ofseismic waves, recording of the signals received by at least one seismicpickup (4) and formation of seismograms by processing the recordedsignals, characterized in that: emission is carried out by coupling withthe formation at least two vibrators (5) emitting simultaneously andcontrolled by orthogonal signals consisting of sinusoids of differentfrequencies, in the fundamental components thereof as well as in therespective harmonics thereof, so as to form a composite vibrationalsignal; and processing comprises discrimination of the respectivecontributions of the vibrators to the composite vibrational signal andreconstruction of seismograms equivalent to those that would be obtainedby actuating the vibrators separately. 2) A method as claimed in claim1, characterized in that orthogonal signals based on wavelets, Legendrepolynomials or random series are emitted. 3) A method as claimed inclaim 1 or 2, characterized in that discrimination of the respectivecontributions of the vibrators is carried out by determining theamplitude and the phase of the composite vibrational signal at thefundamental frequencies of the pilot signals applied to the vibrators.4) A method as claimed in any one of claims 1 to 3, characterized inthat discrimination of the respective contributions of vibrators (5)comprises weighting the recorded signals by a bell weighting factor (13)and determining the amplitude and the phase of the composite signal. 5)A method as claimed in the claim 4, characterized in that discriminationof the respective contributions of the vibrators comprises selection, byFourier transform, of lines (15-18) of the complex spectrum respectivelyassociated with the various weighted signals. 6) A method as claimed inany one of claims 1 to 5, characterized in that reconstruction of theseismograms specifically corresponding to the various vibrators iscarried out by applying, after separation thereof, an inverse Fouriertransform to the lines (19, 20) respectively associated with the variousweighted signals. 7) A method as claimed in any one of the precedingclaims, characterized in that the frequencies of the orthogonal pilotsignals respectively applied to the various vibrators are shifted byfrequency intervals, at predetermined time intervals, so as to sweep acertain emission frequency band [f_(b)−f_(f)]. 8) A system intended forseismic monitoring of an underground formation, comprising emissionmeans allowing emission of seismic vibrations in the formation, meansallowing reception of the signals reflected by the formation in responseto the emission of seismic waves, recording means for recording thesignals received by the signal reception means and processing means forprocessing recorded signals so as to form seismograms, characterized inthat: the emission means comprise at least two vibrators (5) and means(8) for generating orthogonal signals consisting of sinusoids ofdifferent frequencies, in the fundamental components thereof as well asin the respective harmonics thereof, and for applying them respectivelyto vibrators (5) so as to generate in the formation a compositevibrational signal, and the processing means comprise at least onecomputer (6) suited to carry out discrimination of the respectivecontributions of the vibrators to the composite vibrational signal andreconstruction of seismograms equivalent to those that would be obtainedby actuating the vibrators separately. 9) A system as claimed in claim8, characterized in that it comprises a plurality of local units (LU)arranged at a distance from one another and coupled with the formation,each unit comprising at least one seismic pickup (4), a seismic vibrator(5), a local device (6) intended for acquisition and processing of thesignals received, and a central control and synchronization unit (8)connected to the various local units, comprising a signal generatorsuited to apply to vibrators (5) the orthogonal vibrational pilotsignals. 10) A system as claimed in claim 9, characterized in that thecentral control and synchronization unit (8) is connected to the variouslocal units by material or electromagnetic links. 11) A system asclaimed in claim 9 or 10, characterized in that it comprises a pluralityof local units (LU) arranged at a distance from one another and coupledwith the formation, each unit comprising at least one seismic pickup, aseismic vibrator (5), and a central control and synchronization unit (8)connected to the various local units (LU) comprising a signal generatorsuited to form the various orthogonal vibrational pilot signals, andmeans intended for acquisition of the signals received by the variousantennas (2) and for reconstruction of the seismograms corresponding tothe contributions of the various vibrators (5). 12) A system as claimedin claim 10 or 11, characterized in that the reception means comprise atleast one antenna (2) consisting of several seismic pickups (4) arrangedalong a well (3) drilled in the formation, this antenna being connectedto recording means.